Method and installation for helium production

ABSTRACT

The invention concerns a method which consists in cooling the feed mixture, and introducing it in the vessel of a distillation column ( 3 ), partly condensing (in  4 ) the column head gas, cooling (in  5 ) the resulting gas phase so as to condense it partly, and recuperating as impure helium the gas phase resulting from that second condensation. Additionally at least a liquid is produced through a closed refrigerating cycle ( 8 ) with variable flow-rate. The invention is useful for producing helium and liquid nitrogen from a natural gas liquefying residue gas.

[0001] The present invention relates firstly to a process for producingimpure helium from a feed gas mixture containing helium and otherheavier components, of the type in which:

[0002] the feed mixture is brought to a distillation pressure;

[0003] the feed mixture is cooled at this distillation pressure andintroduced into the bottom of a distillation column;

[0004] the overhead gas in the column is partly condensed;

[0005] the resulting gas phase is cooled so as to partially condense it;and

[0006] the gas phase resulting from this second partial condensation isrecovered as impure helium product.

[0007] The invention applies for example to the production of heliumfrom a waste gas coming from a natural gas liquefier, this waste gascontaining, besides helium, methane and nitrogen.

[0008] The pressures involved here are absolute pressures.

[0009] A process of this type is disclosed in U.S. Pat. No. 4,701,200,in which the impure helium is then purified by selective pressure swingadsorption (PSA).

[0010] When the helium has to be purified cryogenically and/orliquefied, there is a need for a cryogenic liquid. The object of theinvention is to meet this need in a particularly effective manner.

[0011] For this purpose, the subject of the invention is a process ofthe aforementioned type, characterized in that at least one of saidcomponents is furthermore produced in the form of a liquid product byimplementing a variable-rate closed refrigeration cycle, the rate ofwhich depends on the output of said liquid product.

[0012] This process may comprise one or more of the following features:

[0013] the refrigeration cycle is a nitrogen cycle;

[0014] the liquid phase obtained by the second partial condensation isenriched with one of said components and the resulting fluid isrecovered as second product;

[0015] said second product is at least partly liquid;

[0016] the partial condensation of the overhead gas in the column iscarried out by heat exchange with pre-expanded liquid from the bottom ofthe column;

[0017] a fraction of the gas resulting from the vaporization of saidexpanded bottom liquid is added to the feed mixture; and

[0018] the distillation pressure is substantially equal to the impurehelium production pressure.

[0019] The subject of the invention is also a unit for implementing theprocess described above. This unit, of the type comprising:

[0020] means for bringing the feed mixture to a distillation pressure;

[0021] a heat exchange line for cooling the feed mixture at thisdistillation pressure;

[0022] a distillation column, the bottom of which is connected to thecold end of the heat exchange line and is fitted with a top condenser;

[0023] a heat exchanger suitable for cooling and partly condensing thevapor phase coming from the condensation passages of the top condenser;

[0024] a phase separator connected to the cold end of the heatexchanger; and

[0025] means for recovering the vapor phase coming from the separator asimpure helium product is characterized in that it furthermore includes avariable-rate closed refrigeration cycle for producing a variable outputof a liquid product.

[0026] Further subjects of the invention are:

[0027] firstly, a pure helium production process, characterized in thatan impure helium production process as defined above is carried out andthe impure helium and said liquid product are sent to a unit forpurifying, and optionally liquefying, helium having a cryogenic part;and

[0028] secondly, a pure helium production installation, characterized inthat it comprises an impure helium production unit as defined above anda unit for purifying, and optionally liquefying, helium containing acryogenic part, this unit being fed with the impure helium produced bythe unit and with said liquid product.

[0029] An example of how the invention is implemented will now bedescribed in conjunction with the appended drawing, the single figure ofwhich shows schematically a helium production installation according tothe invention.

[0030] The installation shown in the drawing comprises an impure heliumproduction unit A and a unit B for purifying the impure helium. Thisunit B has a cryogenic part, which may be a cryogenic heliumpurification apparatus and/or a pure helium liquefaction apparatus.

[0031] The unit A essentially comprises a two-stage feed compressor 1, aheat exchange line 2, a distillation column 3 provided with a topcondenser 4, a heat exchanger 5, two phase separators 6 and 7 and aclosed nitrogen refrigeration cycle 8.

[0032] The unit A produces impure helium at 25 bar (line 9), gaseousnitrogen at 7 bar (line 10), liquid nitrogen at 8 bar (line 11), a firstmethane-rich waste W1 (line 12), a second methane-rich waste W2 (line13) and a nitrogen-rich waste W3 (line 14). All the cold parts of thisunit A are contained in the one cold box 15, which is thermallyinsulated suitably.

[0033] In operation, a low-pressure (3.9 bar) feed mixture, sent via aline 16, is compressed to 8 bar by the first stage of the compressor 1and then compressed to 26 bar that is to say slightly above the impurehelium production pressure, at the same time as a high-pressure feedmixture sent via a line 17.

[0034] The compressed mixture is cooled and partly condensed in theexchange line 2 and introduced into the bottom of the column 3. Theliquid from the bottom of this column is expanded in an expansion valve18 to approximately 4 bar and introduced into the shell 19 of thecondenser 4, in order to refrigerate the latter.

[0035] This refrigeration in the condenser 4 causes the overhead gas inthe column to partly condense. The uncondensed fraction is anintermediate gas containing approximately 25% helium and 75% nitrogen,with traces of methane.

[0036] This intermediate gas is cooled and partly condensed in theexchanger 5 and then separated into two phases in the separator 6.

[0037] The vapor coming from this separator contains approximately 70%helium. After being warmed in 5 and in 2, this vapor constitutes theimpure helium produced by the unit A, recovered, after being warmed in 5and then in 2, via the line 9. This gas will then be treated in the unitB for purifying, and optionally liquefying, helium.

[0038] The methane-rich gas that results from the vaporization in theshell 19, which still contains helium, is warmed in 2. The majority ofthis gas is recycled via a line 20 to the intake of the first stage ofthe compressor 1, while the remainder is discharged via the line 12 asmethane-rich waste W1.

[0039] A stream of the liquid contained in the shell 19 is withdrawn viaa pump 21, vaporized and warmed in the exchange line 2, and thenconstitutes the other methane-rich waste W2, which is discharged via theline 13.

[0040] Of course, the two methane-rich streams may be utilized,especially for their calorific value.

[0041] The liquid phase from the separator 6 contains approximately 99%nitrogen. After expansion to 8 bar in an expansion valve 22, this liquidis introduced into the separator 7. The liquid phase constitutes theliquid nitrogen production by the unit A, which is conveyed via the line11, whereas the gas phase constitutes a nitrogen-rich waste W3,containing approximately 95% nitrogen, which is discharged via the line14.

[0042] As shown, one portion of the liquid nitrogen is vaporized in theexchanger 5 in order to refrigerate the latter, and then, after beingwarmed in 2, it forms the nitrogen gas production stream recovered viathe line 10.

[0043] The nitrogen refrigeration cycle 8, is a closed cycle with avariable rate independent of the rest of the installation. It comprisesa cycle compressor 23 and the combination of a turbine 24 and asupercharger 25 that are mutually coupled.

[0044] The cycle nitrogen, compressed in 23 to 15 bar and superchargedin 25 to 23 bar, is cooled in the exchange line 2 down to anintermediate temperature level and expanded in 24 to 1.6 bar. Thenitrogen thus cooled serves to refrigerate the exchanger 5, thencontinues to be warmed up in 2 and returns to the intake of thecompressor 23.

[0045] The refrigeration cycle is not necessary when liquid nitrogen isproduced, and the stream of cycle nitrogen is regulated according to theoutput of liquid nitrogen produced. This liquid nitrogen is at leastpartly sent via the line 11 to the purification/liquefaction unit B and,optionally, partly withdrawn from the installation via a production line26 tapped off the line 11.

[0046] The unit B, fed with impure helium via the line 9 and with liquidnitrogen via the line 11, produces, via a line 27, pure, possibly liquidhelium and, via a line 28, a waste gas W4.

[0047] Thus, the liquid nitrogen production is integrated into the unitA and especially into its cold box 15.

1-14 (canceled).
 15. A process for producing helium from a feed gasmixture containing helium and other heavier components, comprising: a)pressuring said feed mixture to a distillation pressure; b) cooling saidpressurized feed mixture; c) introducing said feed mixture into thebottom of a distillation column; d) partly condensing the overhead gasin the column; e) cooling the resulting gas phase so as to partiallycondense it; and f) recovering the gas phase resulting from this secondpartial condensation as a helium product, wherein at least one of saidcomponents from said feed gas mixture is further processed to a liquidproduct by implementing a variable-rate closed refrigeration cycle, inwhich the rate of said refrigeration cycle depends on the output of saidliquid product.
 16. The process of claim 15, wherein said refrigerationcycle is a nitrogen cycle.
 17. The process of claim 15, wherein theliquid phase obtained by the second partial condensation is enrichedwith one of said components and in that the resulting fluid is recoveredas a second product.
 18. The process of claim 17, wherein said secondproduct is at least partly liquid.
 19. The process of claim 15, whereinthe partial condensation of said overhead gas in the column is carriedout by heat exchange with pre-expanded liquid from the bottom of thecolumn.
 20. The process of claim 19, wherein a fraction of the gasresulting from the vaporization of said expanded bottom liquid is addedto the feed mixture.
 21. The process of claim 20, wherein thedistillation pressure is substantially equal to the helium productionpressure.
 22. A process for producing helium, further comprising: g)sending said helium product and said liquid product to a second unit forpurifying, and optionally liquefying, helium having a cryogenic part.23. A unit for producing helium from a feed gas mixture containinghelium and other heavier components, comprising: a) means for bringingthe feed mixture to a distillation pressure; b) a heat exchange line forcooling the feed mixture at this distillation pressure; c) adistillation column, the bottom of which is connected to the cold end ofthe heat exchange line and is fitted with a top condenser; d) a heatexchanger suitable for cooling and partly condensing the vapor phasecoming from the condensation passages of the top condenser; e) a phaseseparator connected to the cold end of the heat exchanger; and f) meansfor recovering the vapor phase coming from the separator as a heliumproduct, wherein said unit further comprises a variable-rate closedrefrigeration cycle for producing a variable output of a liquid product.24. The unit of claim 23, wherein said refrigeration cycle is a nitrogencycle.
 25. The unit of claim 23, further comprising means for enrichingthe liquid phase coming from the phase separator with one of saidcomponents and means for recovering the resulting fluid as a secondproduct.
 26. The unit of claim 23, wherein the column further comprisesmeans for expanding its bottom liquid and for sending the expandedliquid into vaporization passages in the top condenser.
 27. The unit ofclaim 26, further comprising a line for recycling the gas contained inthe shell of the top condenser into said feed gas mixture.
 28. Aninstallation for producing pure helium comprising: a) a first unit forproducing helium comprising: 1) means for bringing the feed mixture to adistillation pressure; 2) a heat exchange line for cooling the feedmixture at this distillation pressure; 3) a distillation column, thebottom of which is connected to the cold end of the heat exchange lineand is fitted with a top condenser; 4) a heat exchanger suitable forcooling and partly condensing the vapor phase coming from thecondensation passages of the top condenser; 5) a phase separatorconnected to the cold end of the heat exchanger; and 6) means forrecovering the vapor phase coming from the separator as a heliumproduct, wherein said unit further comprises a variable-rate closedrefrigeration cycle for producing a variable output of a liquid product;and b) a second unit for purifying, and optionally liquefying, heliumhaving a cryogenic part, wherein said second unit is fed with the heliumproduced by the first unit and with said liquid product.